System and method for correcting for image artifacts in X-ray image data

ABSTRACT

A computer-implemented method for reducing image artifacts in X-ray image data includes dividing pixels of X-ray image data into a plurality of pixel value regions based on a pixel value of each pixel, wherein each pixel value region has a different range of pixel values. The method also includes generating calibrated X-ray image data for each pixel value region, wherein the respective calibrated X-ray image data for each pixel value region is generated using a different dose of radiation. Further, the method includes calculating a gain slope for each pixel value region based on the calibrated X-ray image data, and calculating a pixel gain correction for the pixels of the X-ray image data based on at least one of the calculated gain slopes.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of U.S. patentapplication Ser. No. 13/588,832, entitled “SYSTEM AND METHOD FORCORRECTING FOR IMAGE ARTIFACTS IN X-RAY IMAGE DATA”, filed Aug. 17,2012, which is herein incorporated by reference in its entirety.

BACKGROUND

The subject matter disclosed herein relates to X-ray imaging systems andmore particularly to correcting for image artifacts in X-ray image datain X-ray imaging systems.

Digital X-ray imaging systems are becoming increasingly widespread forproducing digital data which can be reconstructed into usefulradiographic images. In current digital X-ray imaging systems, radiationfrom a source is directed toward a subject, typically a patient in amedical diagnostic application. A portion of the radiation passesthrough the patient and impacts a detector. The scintillator of thedetector converts the radiation to light photons that are sensed. Thedetector is divided into a matrix of discrete picture elements orpixels, and encodes output signals based upon the quantity or intensityof the radiation impacting each pixel region, as measured by a receptor.The signals may then be processed to generate an image that may bedisplayed for review.

The relationship between input intensity of the radiation impacting thedetector and detector output may vary between the pixels of thedetector. Calibration of the detector can correct for such variations.However, the relationship between intensity of radiation impacting thedetector and detector output can be difficult to calibrate across acertain dynamic range of X-ray doses that may be applied to thedetector.

BRIEF DESCRIPTION

In one embodiment of the present disclosure, a computer-implementedmethod for reducing image artifacts in X-ray image data includesdividing pixels of X-ray image data into a plurality of pixel valueregions based on a pixel value of each pixel, wherein each pixel valueregion has a different range of pixel values. The method also includesgenerating calibrated X-ray image data for each pixel value region,wherein the respective calibrated X-ray image data for each pixel valueregion is generated using a different dose of radiation. Further, themethod includes calculating a gain slope for each pixel value regionbased on the calibrated X-ray image data, and calculating a pixel gaincorrection for the pixels of the X-ray image data based on at least oneof the calculated gain slopes.

In another embodiment of the present disclosure, one or morenon-transitory computer-readable media encoding one or moreprocessor-executable routines are provided. The routines, when executedby a processor, cause acts to be performed, including dividing pixels ofX-ray image data into a plurality of pixel value regions based on apixel value of each pixel, wherein each pixel value region has adifferent range of pixel values. The acts performed by the routines alsoinclude generating calibrated X-ray image data for each pixel valueregion, wherein the respective calibrated X-ray image data for eachpixel value region is generated using a different dose of radiation.Further, the routines perform acts including calculating a gain slopefor each pixel value region based on the calibrated X-ray image data andcalculating a pixel gain correction for the pixels of the X-ray imagedata based on at least one of the calculated gain slopes.

In a further embodiment of the present disclosure, an imaging systemincludes a radiation source, a digital X-ray detector configured togenerate X-ray image data, and control circuitry configured to apply again correction to the X-ray image data via a plurality of slope maps.The processing circuitry is configured to generate the slope maps byexecuting code to perform the act of dividing pixels of X-ray image datainto a plurality of pixel value regions based on a pixel value of eachpixel, wherein each pixel value region has a different range of pixelvalues. The executed code also performs the acts of generatingcalibrated X-ray image data for each pixel value region, wherein therespective calibrated X-ray image data for each pixel value region isgenerated using a different dose of radiation, and calculating a gainslope for each pixel value region based on the calibrated X-ray imagedata.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentdisclosure will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is perspective view of an embodiment of a fixed X-ray system,equipped in accordance with aspects of the present disclosure;

FIG. 2 is a perspective view of an embodiment of a mobile X-ray system;

FIG. 3 is a diagrammatical overview of an embodiment of an X-ray system;

FIG. 4 is a plot illustrating an embodiment of pixel output versus X-raydose;

FIG. 5 is a plot illustrating an embodiment of pixel gain correctionversus X-ray dose;

FIG. 6 is a process flow diagram of an embodiment of a method forcorrecting for image artifacts in an X-ray system;

FIG. 7 is a process flow diagram of an embodiment of a method forcalculating a gain slope for a pixel value region; and

FIG. 8 is a process flow diagram of an embodiment of a method forcalculating a pixel gain correction.

DETAILED DESCRIPTION

Present embodiments are directed to systems and methods for correctingfor image artifacts in X-ray image data. Such image artifacts may be aresult of a nonlinear relationship between the dose of radiation appliedto an X-ray detector and the pixel values output by the detectorelements. The X-ray imaging system is designed to divide X-ray imagedata output by the detector elements into multiple pixel value regions.The pixel value regions have different ranges of pixel values separatedby pixel value boundaries. Once divided into multiple pixel valueregions, a dose of radiation may be applied to the detector forcalibration of each pixel value region. This enables the detector togenerate calibrated X-ray image data for each pixel value region inresponse to the applied X-ray dose. A processor of the X-ray system maycalculate a linear gain slope for each of the pixel value regions fromaverage values of the calibrated X-ray image data for each region. Usingthese gain slopes, the processor may determine a gain correction valueto apply to each detector element based on the pixel output of thedetector and the pixel value region in which the pixel output islocated. In this way, the X-ray system performs a simple correction ofX-ray image data based on linear slope maps developed from calibratingthe detector across different ranges of pixel values.

Referring generally to FIG. 1, an imaging system, in particular an X-raysystem is represented and referenced generally by reference numeral 10.In the illustrated embodiment, the X-ray system 10 is a digital X-raysystem. The X-ray system 10 is designed both to acquire original imagedata and to process the image data for display in accordance withpresent techniques. The X-ray system 10 may be a radiographic imagingsystem used to collect a standalone image, or a fluoroscopic imagingsystem used to collect multiple frames of real-time image data. In theembodiment illustrated in FIG. 1, the X-ray system 10 includes an imagersystem 12. The imager system 12 includes an overhead tube support arm 14for positioning a radiation source 16, such as an X-ray tube, and acollimator 18 with respect to a patient 20 and a portable digital X-raydetector 22. In one embodiment, the imager system 12 may be used inconsort with one or both of a patient table 26 and a wall stand 28 tofacilitate image acquisition. Particularly, the table 26 and the wallstand 28 may be configured to receive the detector 22. For instance, thedetector 22 may be placed on an upper, lower, or intermediate surface ofthe table 26, and the patient 20 (more specifically, an anatomy ofinterest of the patient 20) may be positioned on the table 26 betweenthe detector 22 and the radiation source 16. The wall stand 28 mayinclude a receiving structure 30 also adapted to receive the detector22, and the patient 20 may be positioned adjacent the wall stand 28 toenable the image data to be acquired via the detector 22. The receivingstructure 30 may be moved vertically along the wall stand 28.

Also depicted in FIG. 1, the imager system 12 includes a systems cabinet31 that includes a workstation 32 and display 34. In one embodiment, theworkstation 32 may include or provide the functionality of the imagersystem 12 such that a user, by interacting with the workstation 32, maycontrol operation of the source 16 and detector 22. The detector 22 maybe in communication with the workstation 32 as described below. Theworkstation 32 may house systems electronic circuitry that acquiresimage data from the detector 22 and that, where properly equipped (e.g.,when the workstation 32 includes processing circuitry), may process thedata to form desired images. In addition, the systems electroniccircuitry both provides and controls power to the X-ray source 16. Theworkstation 32 may include buttons, switches, or the like to facilitateoperation of the X-ray source 16 and detector 22. In one embodiment, theworkstation 32 may be configured to function as a server of instructionsand/or content on a network 36 of the medical facility, such as ahospital information system (HIS), a radiology information system (RIS),and/or picture archiving communication system (PACS). In certainembodiments, the workstation 32 and/or detector 22 may wirelesslycommunicate with the network 36.

In present embodiments, the detector 22 includes circuitry forprocessing the image data acquired through the detector 22 beforecommunicating the processed image data to the workstation 32. Thedetector 22 may have a nonlinear relationship between the dose of X-rayradiation received by the detector 22 and the signals output by thepixels of the detector 22. To account for this nonlinearity, theprocessing circuitry may apply a gain correction to the signals beforethey are communicated to the workstation 32 for storing or viewing. Thedetector 22 may be calibrated at different doses of radiation, and theprocessing circuitry may execute steps to calculate a pixel gaincorrection for each receptor outputting pixel values within a certainrange. That is, the detector 22 may be calibrated separately for eachpixel value range before images are produced by the X-ray system 10. Inother embodiments, such calibration and processing of the image dataacquired through the detector 22 may be performed via processingcircuitry in the workstation 32 after the image data is communicatedfrom the detector 22 to the workstation 32. In certain embodiments, thedetector 22 may be used in a radiographic X-ray imaging system, while inother embodiments the detector 22 may be used in a fluoroscopic X-rayimaging system. In either context, the presently described calibrationand image processing techniques may be applied to the X-ray system 10for correcting nonlinearity in the detector 22.

In one embodiment, the imager system 12 may be a stationary systemdisposed in a fixed X-ray imaging room, such as that generally depictedin and described above with respect to FIG. 1. It will be appreciated,however, that the presently disclosed techniques may also be employedwith other imaging systems, including mobile X-ray units and systems, inother embodiments.

For instance, as illustrated in the medical imaging system 10 (e.g.,X-ray system) of FIG. 2, the imager system 12 may be moved to a patientrecovery room, an emergency room, a surgical room, or any other space toenable imaging of the patient 20 without requiring transport of thepatient 20 to a dedicated (i.e., fixed) X-ray imaging room. The X-raysystem 10 includes a mobile imager or mobile X-ray base station 50 and aportable digital X-ray detector 22. As above, the illustrated X-raysystem 10 is a digital X-ray system. In one embodiment, a support arm 52may be vertically moved along a support column 54 to facilitatepositioning of the radiation source 16 and collimator 18 with respect tothe patient 20. Further, one or both of the support arm 52 and supportcolumn 54 may also be configured to allow rotation of the radiationsource 16 about an axis. In addition, the X-ray base station 50 has awheeled base 58 for movement of the station 50.

The patient 20 may be located on a bed 60 (or gurney, table or any othersupport) between the X-ray source 24 and the detector 22 and subjectedto X-rays that pass through the patient 20. During an imaging sequenceusing the digital X-ray system 10, the detector 22 receives X-rays thatpass through the patient 20 and transmits imaging data to a base unit56. The detector 22 is in wireless communication with the base unit 56.The base unit 56 houses systems electronic circuitry 62 that acquiresimage data from the detector 22 and that, where properly equipped, mayprocess the data to form desired images. In addition, the systemselectronic circuitry 62 both provides and controls power to the X-raysource 16 and the wheeled base 58. The base unit 56 also has theoperator workstation 32 and display 34 that enables the user to operatethe X-ray system 10. The operator workstation 32 may include buttons,switches, or the like to facilitate operation of the X-ray source 16 anddetector 22. In one embodiment, the workstation 32 may be configured tofunction as a server of instructions and/or content on the network 36 ofthe medical facility, such as HIS, RIS, and/or PACS. In certainembodiments, the workstation 32 and/or detector 22 may wirelesslycommunicate with the network 36.

Similar to the X-ray system 10 in FIG. 1, components of the imagersystem 12 (e.g., base unit 56) and the detector 22 are configured toperform a correction of the X-ray image data based on a multi-partcalibration. That is, multiple calibration images are obtained by thedetector 22 to determine receptor outputs at different radiation doses.From the calibration images, a number of slope maps may be generated,each slope map corresponding with a different range of pixel outputvalues. A gain correction for X-ray image data may then be determinedbased on one or more of the slope maps generated through thiscalibration, depending on the pixel value region in which a pixel of theX-ray image data is located. The generated slope maps may be utilized toprovide corrected X-ray image data in a variety of mobile X-ray systems10, including radiographic imaging systems and fluoroscopic imagingsystems.

Regardless of the differences between the X-ray systems 10 shown inFIGS. 1 and 2, certain features internal to the X-ray system 10 remainconsistent across different embodiments. These components areillustrated diagrammatically in FIG. 3. The imager system 12 includesthe X-ray source 16 of radiation. The X-ray source 16 is controlled by apower supply 70, which furnishes both power and control signals forexamination sequences. In addition, in mobile imaging systems the powersupply 70 furnishes power to a mobile drive unit 72 of the wheeled base58. The power supply 70 is responsive to signals from a systemcontroller 74. In general, the system controller 74 commands operationof the imaging system to execute examination protocols and to processacquired image data. In the present context, the system controller 74also includes signal processing circuitry, typically based upon ageneral purpose or application-specific digital computer, associatedmemory circuitry for storing programs and routines executed by thecomputer, as well as configuration parameters and image data, interfacecircuits, and so forth. The system controller 74 may include or may beresponsive to a processor 76. The processor 76 receives image data fromthe detector 22 and processes the data to reconstruct an image of asubject.

The processor 76 is linked to a wireless communication interface 80 thatallows wireless communication with the detector 22. Further, theprocessor 76 is linked to a wired communication interface 82 that allowscommunication with the detector 22 via a tether (e.g., a multi-conductorcable). The imager system 12 may also be in communication with a server.The processor 76 is also linked to a memory 84, an input device 86, andthe display 34. The memory 84 stores configuration parameters,calibration files received from the detector 22, and lookup tables usedfor image data processing. The input device 86 may include a mouse,keyboard, or any other device for receiving user input, as well as toacquire images using the imager system 12. The display 34 allowsvisualization of output system parameters, images, and so forth.

The detector 22 includes a wireless communication interface 88 forwireless communication with the imager system 12, as well as a wiredcommunication interface 90, for communicating with the detector 22 whenit is tethered to the imager system 12. The detector 22 may also be incommunication with a server. It is noted that the wireless communicationinterface 88 may utilize any suitable wireless communication protocol,such as an ultra wideband (UWB) communication standard, a Bluetoothcommunication standard, or any 802.11 communication standard. Moreover,the detector 22 is coupled to a detector controller 92 which coordinatesthe control of the various detector functions. For example, the detectorcontroller 92 may execute various signal processing and filtrationfunctions, such as for initial adjustment of dynamic ranges,interleaving of digital image data, and so forth. The detectorcontroller 92 is responsive to signals from the system controller 74, aswell as the detection circuitry 78. The detector controller 92 is linkedto a processor 94. The processor 94, the detector controller 92, and allof the circuitry receive power from a power supply 96. The power supply96 may include a battery. In some embodiments, the detector 22,including the power supply 96, may receive power from the power supply70 when tethered to the imager system 12.

Also, the processor 94 is linked to detector interface circuitry 98. Thedetector 22, which again may be used in radiographic or fluoroscopicimaging systems, converts X-ray photons received on its surface to lowerenergy photons. The detector 22 includes a detector array 100 thatincludes an array of photodetector elements to convert the light photonsto electrical signals, which are representative of the number of photonsor the intensity of radiation impacting individual pixel regions of thedetector surface. Alternatively, the detector 22 may convert the X-rayphotons directly to electrical signals. These electrical signals areconverted to digital values by the detector interface circuitry 98,which provides the values to the processor 94 to be converted to imagingdata and sent to the imager system 12 to reconstruct an image of thefeatures within a subject. Alternatively, the imaging data may be sentfrom the detector 22 to a server to process the imaging data. In apresent form, the detector array 100 is formed of silicon complimentarymetal-oxide-semiconductors (CMOS). The array elements, or receptors, areorganized in rows and columns, with each receptor consisting of aphotodiode and complementary and symmetrical pairs of p-type and metaloxide semiconductor field effect transistors (MOSFET). The cathode ofeach diode is connected to the source of the transistor, and the anodesof all diodes are connected to a negative bias voltage. The gates of thetransistors in each row are connected together and the row electrodesare connected to scanning electronics. The drains of the transistors ina column are connected together and the electrode of each column isconnected to an individual data channel of the detector interfacecircuitry 98.

Further, the processor 94 is linked to a memory 104. The memory 104 maystore various configuration parameters, calibration files, and detectoridentification data. In addition, the memory 104 may store slope mapsused to determine a pixel correction applied to the image data collectedby the detector 22. These slope maps may be generated based oncalculations discussed in detail below. In some embodiments, these slopemaps may be determined via the processor 76 and stored in the memory 84of the imager system 12, and not the detector 22 itself. That is, eitherthe imager system 12 or the detector 22 may store and process thecalibration data collected by the detector 22.

Each receptor of the detector array 100 may output a different signalvalue to the processor 94 in response to receiving a certain dose ofradiation. That is, the relationship between X-ray dose and receptoroutput may be different for each receptor. In order for the signalsreceived from each of the receptors to form an accurate X-ray image, thedetector 22 may first be calibrated to determine a gain correction toapply to X-ray image data that is later collected using the detector 22.In present embodiments, the dynamic range of the detector 22 isrelatively small, such that the detector 22 uses the entire availabledynamic range. As a result, there may be inherent non-linearity in therelationship between X-ray dose and receptor output for any givenreceptor. As a result, the pixel value of any pixel within the detectorarray 100 may vary nonlinearly with respect to the X-ray dose applied tothe pixel. Calibrating such a detector 22 at a single X-ray dose ofradiation may yield a gain correction map that, when applied to otherX-ray image data sets, would introduce shape artifacts into thecorrected X-ray image. To reduce these artifacts, the detector 22 may beconfigured to perform a separate calibration across multiple pixel valueregions within the receptor's dynamic range.

To demonstrate this method of calibration, FIG. 4 is a plot 110illustrating an embodiment of pixel output 112 versus X-ray dose 114.The pixel output 112 is related to the signal generated by a pixel ofthe detector array 100 when a certain dose 114 of radiation is appliedto the detector 22. Five traces 116 shown on the plot 110 arerepresentative of five pixels, located at various spatial locationswithin the detector array 100. The illustrated traces 116 show anonlinear relationship between pixel output 112 and X-ray dose 114 foreach of their respective pixels of the detector array 100.

To account for such nonlinearity during image correction, the plot 110shows the pixel output 112 divided into sections, or pixel valueregions. Each pixel value region has a different range of pixel values.In the illustrated embodiment, the pixel output 112 is divided intothree pixel value regions 118, 120, and 122. The pixel output 112 may bedivided into any number of different pixel value regions, depending onthe relationship of the pixel output 112 to the X-ray dose 114,acceptable calibration error, and other factors. The pixel value regions118, 120, and 122 each have at least one pixel value boundary separatingvalues of the pixels within a respective pixel value region from pixelsvalues within another pixel value region. For example, in theillustrated plot 110, a pixel value boundary 124 indicates the dividingline between the pixel value regions 118 and 120. Similarly, anotherpixel value boundary 126 divides the pixel value regions 120 and 122.Because there are two pixel value boundaries 124 and 126, theillustrated pixel gain model is bilinear. It should be noted that thepixel value boundaries 124 and 126 are the same for each pixel in thedetector array 100. The first pixel value region 118 includes all pixeloutputs 112 between 0 and p^({1}) (e.g., 124), the second pixel valueregion 120 includes all pixel outputs 112 between p^({1}) (e.g., 124)and p^({2}) (e.g., 126), and the third pixel value region 122 includesall pixel outputs 112 above the value of p^({2}) (e.g., 126). Theprocessor 94 may determine the appropriate pixel value boundaries 124and 126, e.g., based on the available dynamic range of the detectorarray 100, by implementing machine-readable code stored in the memory104. In certain embodiments, a user may select the pixel valueboundaries 124 and 126 via the input device 86, and the processor 94 mayreceive and determine the division of pixel values based on this userselection. It should be noted that there may be any number of pixelvalue boundaries selected in order to divide the pixels of X-ray imagedata into a desired number of pixel value regions for calibration andcorrection of X-ray image data.

Once the pixel output 112 is divided into a number of pixel valueregions having different ranges of pixel values, an image calibrationmay be performed for each pixel value region. The calibration mayinclude applying a certain dose 114 of radiation to the detector 22 andstoring the resulting pixel outputs 112 from each pixel of the detectorarray 100. The illustrated plot 110 shows three different doses 128,130, and 132 of radiation applied to the pixel value regions 118, 120,and 122, respectively. These doses 128, 130, and 132 may be selectedsuch that the pixel output 112 for the different pixels at each dose aresubstantially central within the range of pixel values for therespective pixel value region. To reduce noise, multiple images may beacquired at each of the doses 128, 130, and 132, and the images may beaveraged together to create gain maps for each of the pixel valueregions 118, 120, and 122, as discussed in detail below. In someembodiments, the X-ray system 10 may be used to determine each of thedoses 128, 130, and 132 of radiation, apply the doses 128, 130, and 132,acquire the pixel outputs 112 at each dose, and select the pixel valueboundaries 124 and 126 based on the pixel outputs 112.

From the pixel outputs 112 collected in response to applying each doseof radiation to the detector array 100, an average gain calibrationimage may be determined for each dose. The calibration image includesthe pixel output 112 of each detector pixel in response to a certainradiation dose. Multiple calibration images may be collected for thesame X-ray dose, and the image data for each calibration image averagedtogether to yield the average calibration image. For a detector 22divided into three pixel value regions 118, 120, and 122, as shown inthe illustrated embodiment, the average calibration image for eachrespective region may be denoted as {p_(i,j) ^({1})}, {p_(i,j) ^({2})},and {p_(i,j) ^({3})}.

FIG. 5 is a plot 140 illustrating an embodiment of pixel gain correction142 as a function of X-ray dose 114. The pixel gain correction 142, oncedetermined, may be applied to X-ray image data collected by the detector22, in order to reduce any image artifact caused by nonlinearity in theX-ray pixels or detector elements. In this way, the pixel gaincorrection 142 relates to the pixel outputs 112 discussed with respectto FIG. 4. The pixel gain correction 142 is applied in three linearsections in the illustrated embodiment, representing the same threepixel value regions 118, 120, and 122 of FIG. 4. These pixel valueregions 118, 120, and 122 are separated by pixel gain value boundaries144 and 146, which are the pixel gain values corresponding to the pixelvalue boundaries 124 and 126, respectively. In calibrating the detector22, the average pixel gain correction 142 is determined for each of thedoses 128, 130, and 132 of radiation applied to the detector 22. Again,these doses 128, 130, and 132 correspond with pixel outputs 112relatively central to the pixel value regions 118, 120, and 122. Asdiscussed in detail below, the processor 94 of the detector 22 maydetermine gain slopes 148, 150, and 152 of the pixel correction 142 forthe respective pixel value regions 118, 120, and 122. These gain slopes148, 150, and 152 may be respectively referred to as s_(i,j) ^({1}),s_(i,j) ^({2}), and s_(i,j) ^({3}). In certain embodiments, the gainslopes 148, 150, and 152 may be determined according to the followingequations:

$\begin{matrix}{{s_{i,j}^{\{ 1\}} = \frac{\alpha \times {median}\{ p_{i,j}^{\{ 1\}} \}}{p_{i,j}^{\{ 1\}}}},} & (1) \\{{s_{i,j}^{\{ 2\}} = \frac{\alpha \times {median}\{ {p_{i,j}^{\{ 2\}} - p^{\{ 1\}}} \}}{p_{i,j}^{\{ 2\}} - p^{\{ 1\}}}},{and}} & (2) \\{s_{i,j}^{\{ 3\}} = {\frac{\alpha \times {median}\{ {p_{i,j}^{\{ 3\}} - p^{\{ 2\}}} \}}{p_{i,j}^{\{ 3\}} - p^{\{ 2\}}}.}} & (3)\end{matrix}$

In the first equation, used to calculate the gain slope of the firstpixel value region 118, median {p_(i,j) ^({1})} represents a median ofthe average gain calibration image for the first pixel value region 118.This may be determined by generating calibration images at one or moreX-ray doses 114 yielding pixel outputs 112 within the first pixel valueregion 118. Again, such calibration images may themselves be an averageimage taken from a number of calibration images generated using the samedose 114 of radiation. The pixel value p_(i,j) ^({1}) represents thepixel output 112 corresponding with the median of the average gaincalibration image. For example, if one dose (e.g., 128) of radiation isapplied for the first pixel value region 118, p_(i,j) ^({1}) would bethe average pixel output of a pixel at a certain location, and median{p_(i,j) ^({1})} would be the average gain calibration image at thelocation. Since a lower pixel value boundary of the first pixel valueregion 118 is 0, both median {p_(i,j) ^({1})} and p_(i,j) ^({1}) arerelative values in Equation (1). Similarly, median {p_(i,j)^({2})−p_({1})} and p_(i,j) ^({2})−p^({1}) are relative with respect tothe lower pixel value boundary 124 of the second pixel value region 120.Likewise, median {p_(i,j) ^({3})−p^({2})} and p_(i,j) ^({3})−p^({2}) arerelative with respect to the lower pixel value boundary 126 of the thirdpixel value region 122.

In the equations shown above, α represents a scaling factor applied tothe gain slope calculations. The scaling factor α may be determined suchthat a maximum pixel value of the gain corrected image is near a desiredmaximum count level when a maximum dose 114 of radiation is applied tothe detector 22. The desired maximum count level may be an upper pixelvalue limit based on an available bit-width of the image data. Forexample, the pixel values may vary from 0 to 16383 in a system thatgenerates 14-bit digital image data. Thus, the scaling factor α may becalculated by the processor 94 such that the upper pixel value limit(e.g., 16383) corresponds to the gain corrected pixel value for a pixelreceiving the maximum available X-ray dose 114 used in the X-ray system10. This enables the corrected X-ray image data to span a full range ofavailable digital pixel values.

The gain slopes 148, 150, and 152 may be calculated from the averagecalibration images associated with the pixel value regions 118, 120, and122. Once calculated, the gain slopes 148, 150, and 152 may be used todetermine the pixel gain correction 142 to be applied to image data atany X-ray dose 114. For example, the gain slope 148 may be used tointerpolate an appropriate gain correction 142 for any pixel of imagedata that is within the first pixel value region 118. In an embodimenthaving three pixel value regions 118, 120, and 122, such as FIG. 6, thepixel gain correction may be calculated by the processor 94 according tothe following equations:

${\hat{p}}_{i,j} = \{ \begin{matrix}{{{p_{i,j} \times s_{i,j}^{\{ 1)}},}\mspace{419mu}} & {{{{if}\mspace{14mu} p_{i,j}} \leq {p^{\{ 1\}}\mspace{76mu}(4)}}\;} \\{{{{p^{\{ 1\}} \times s_{i,j}^{\{ 1)}} + {( {p_{i,j} - p^{\{ 1\}}} ) \times s_{i,j}^{\{ 2)}}},}\mspace{211mu}} & {{{if}\mspace{14mu} p^{\{ 1\}}} < p_{i,j} \leq {p^{\{ 2\}}(5)}} \\{{{p^{\{ 1\}} \times s_{i,j}^{\{ 1)}} + {( {p^{\{ 2\}} - p^{\{ 1\}}} ) \times s_{i,j}^{\{ 2)}} + {( {p_{i,j} - p^{\{ 2\}}} ) \times s_{i,j}^{\{ 3)}}},} & {{{if}\mspace{14mu} p^{\{ 2\}}} < {p_{i,j}.\mspace{65mu}(6)}}\end{matrix} $

In the above equations, p_(i,j) represents a pixel output measured atthe i, j position of the detector array 100. After determining theappropriate gain correction 142 for a pixel based on the pixel output112, the gain correction 142 may be applied to the pixel to reduceartifacts in the image data caused by detector nonlinearity. In someembodiments, the gain correction 142 may be a value subtracted from theX-ray image data collected for each of the pixels on the detector array100. However, other embodiments of the detector 22 may apply the gaincorrection 142 in different ways to generate corrected X-ray image databased on the gain slope maps.

FIG. 6 is a process flow diagram of a method 160 for correcting forimage artifacts in the X-ray system 10. It should be noted that themethod 160 may be a computer-implemented method performed via one ormore processor-executable routines encoded on one or more non-transitorycomputer-readable media (e.g., memory 104). When executed by theprocessor 94, these routines cause the acts shown in the method 160 tobe performed. Specifically, this method 160 enables the processor 94(i.e., processing circuitry) to generate multiple slope maps used toapply a gain correction to the X-ray image data collected aftercalibration of the detector 22. The term non-transitory merely indicatesthat the medium is not a signal.

The method 160 includes determining (block 162) pixel value boundaries(e.g., 124, 126) of pixel value regions (e.g., 118, 120, 122) of thepixel output 112, and dividing (block 164) the pixels of the X-ray imagedata into at least two pixel value regions (e.g., 118, 120, 122). Aspreviously discussed, each pixel value region has a different range ofpixel values, and the pixels of X-ray image data are divided based onthe value of each pixel. The pixel value boundaries separate the valuesof pixels in each of the pixel value regions, and these boundaries maybe determined based on a user selection. The method 160 may includeselecting (block 166) the dose (e.g., 128, 130, 132) of radiation foreach pixel value region (e.g., 118, 120, 122) such that a receptoroutput at each radiation dose is substantially central within therespective pixel value region. The method 160 further includesgenerating (block 168) calibrated X-ray image data for each pixel valueregion (e.g., 118, 120, 122). This may refer to generating one or moreaverage calibration images for each pixel value region, where everycalibration image is generated using a different dose 114 of radiation.

The method 160 includes calculating (block 170) a gain slope (e.g., 148,150, 152) for each pixel value region (e.g., 118, 120, 122) based on thecalibrated X-ray image data. An example of one such calculation is givenabove in Equations (1)-(3), and is described with reference to FIG. 7below. However, other calculations may be possible for determining thegain slopes. The method 160 also includes calculating (block 172) apixel gain correction 142 for the pixels of the X-ray image data basedon at least one of the calculated gain slopes (e.g., 148, 150, 152). Thedetails of one such calculation are given in Equations (4)-(6) above anddescribed with reference to FIG. 8 below. In addition, the method 160may include subtracting (block 174) the pixel gain correction from X-rayimage data collected by the detector 22 to generate a corrected X-rayimage. The detector 22 may communicate the corrected X-ray image to theimager system 12 of the X-ray system 10 for storage in the memory 84 orvisualization on the display 34.

FIG. 7 is a process flow diagram of a method for calculating (block 170)the gain slope (e.g., 148, 150, 152) for a pixel value region (e.g.,118, 120, 122). This includes determining (block 176) a lower pixelvalue boundary (e.g., 0, p^({1}), p^({2})) for the pixel value region.The method also includes determining (block 178) the median value of theaverage gain calibration image (e.g., {p_(i,j) ^({1})}, {p_(i,j)^({2})}, {p_(i,j) ^({3})}) for the respective pixel value region. Themethod further includes multiplying (block 180) the median value by ascaling factor (e.g., α) to generate a scaled median value, and dividing(block 182) the scaled median value by a difference between a medianpixel value (e.g., p_(i,j) ^({1}), p_(i,j) ^({2}), p_(i,j) ^({3})),which corresponds with the median value of the average gain calibrationimage, and the lower pixel value boundary. This method represents thecalculation performed in Equations (1)-(3) above, and may be used todetermine the gain slope for each pixel value region.

FIG. 8 is a process flow diagram of a method for calculating (block 172)the pixel gain correction (e.g., {circumflex over (p)}_(i,j)) for apixel based on its measured pixel output 112 and the pixel value region(e.g., 118, 120, 122) that contains this pixel output 112. This methodincludes multiplying (block 184) the gain slope (e.g., 148, 150, 152) ofthe respective pixel value region by a relative pixel value to determinea relative pixel gain correction. This relative pixel value refers tothe measured pixel value (e.g., p_(i,j)) minus the lower pixel valueboundary (e.g., 0, p^({1}), p^({2})) of the pixel value region. Themethod also includes adding (block 186) the relative pixel gaincorrection to a full pixel gain correction of all other pixel valueregions with pixel values below the lower pixel value boundary. Forexample, since there are no pixel value regions having lower pixelvalues than the first pixel value region 118, the relative pixel gaincorrection p_(i,j)×s_(i,j) ^({1)) is the pixel gain correction, as shownin Equation (4). In the second pixel value region 120, however, therelative pixel gain correction (p_(i,j)−p^({1}))×s_(i,j) ^({2)) is addedto the full pixel value correction p^({1})×s_(i,j) ^({1)) of the firstpixel value region 118, as shown in Equation (5). Similarly, in thethird pixel value region 122, the relative pixel gain correction(p_(i,j)−p^({2}))×s_(i,j) ^({3)) is added to the full pixel valuecorrection p^({1})×s_(i,j) ^({1)) of the first pixel value region 118and the full pixel value correction (p^({2})−p^({1}))×s_(i,j) ^({2)) ofthe second pixel value region 120, as shown in Equation (6). Othercalculations may be performed to determine a pixel gain correction to beapplied for any of the pixel value regions 118, 120, and 122 based on atleast the gain slopes determined from the multiple calibration imagestaken at different doses of radiation.

Technical effects of the disclosed embodiments include, among otherthings, a method of calibrating a digital X-ray detector (used inradiographic or fluoroscopic X-ray imaging systems) that makes full useof a relatively small dynamic range. This would result in image dataartifacts in X-ray systems that determine gain corrections using aone-part linear calibration scheme. Instead, present embodiments providea gain correction that reduces such artifacts based on simplecalculations performed for a number (two or more) of different pixelvalue regions. By dividing the pixel outputs of the detector array intomultiple pixel value regions, each pixel value region may be calibratedseparately to determine a gain slope representative of the average gaincalibration image for that particular range of pixel values. After thiscalibration is performed for each region, the gain correction may bedetermined according to a simple equation based on the gain slopes andpixel outputs of the detector array. This may allow for relativelysimple calibration and correction of X-ray image data collected viareceptors with nonlinear relationships between X-ray dose and pixeloutput. Since the level of calibration is defined based on the pixeloutput of the receptor, the image correction is relativelystraightforward to implement. The calculations are linear, making themcomputationally simple for the processor to apply across all pixels ofthe detector array. This may save image processing time while extendingthe use of the detector's dynamic range.

This written description uses examples to disclose present embodiments,including the best mode, and also to enable any person skilled in theart to practice the embodiments, including making and using any devicesor systems and performing any incorporated methods. The patentable scopeof the disclosure is defined by the claims, and may include otherexamples that occur to those skilled in the art. Such other examples areintended to be within the scope of the claims if they have structuralelements that do not differ from the literal language of the claims, orif they include equivalent structural elements with insubstantialdifferences from the literal languages of the claims.

The invention claimed is:
 1. A computer-implemented method for reducingimage artifacts in X-ray image data, comprising: dividing pixels ofX-ray image data into a plurality of pixel value regions based on apixel value of each pixel, wherein each pixel value region has adifferent range of pixel values; generating calibrated X-ray image datafor each pixel value region, wherein the respective calibrated X-rayimage data for each pixel value region is generated from image dataacquired during exposure to a different dose of radiation; calculating again slope for each pixel value region based on the calibrated X-rayimage data; and calculating a pixel gain correction for the pixels ofthe X-ray image data based on at least one of the calculated gainslopes.
 2. The method of claim 1, wherein each of the pixel valueregions has at least one pixel value boundary separating values ofpixels within the respective pixel value region from other values ofpixels within another pixel value region.
 3. The method of claim 2,wherein dividing the pixels of X-ray image data comprises determiningthe pixel value boundaries based on a user selection.
 4. The method ofclaim 1, comprising selecting the dose of radiation for each pixel valueregion such that a receptor output at each radiation dose issubstantially central within the respective pixel value region.
 5. Themethod of claim 4, wherein generating the calibrated X-ray image datacomprises: acquiring a plurality of images from a digital X-ray detectorfor each selected dose of radiation; and averaging the plurality ofimages acquired at each selected dose to determine an average gaincalibration image for each pixel value region.
 6. The method of claim 1,wherein calculating the pixel gain correction comprises: multiplying thegain slope by a relative pixel value to determine a relative pixel gaincorrection, wherein the relative pixel value is a pixel value within therespective pixel value region minus a lower pixel value boundary of therespective pixel value region; and adding the relative pixel gaincorrection to a full pixel gain correction of all other pixel valueregions having pixel values below the lower pixel value boundary.
 7. Themethod of claim 1, comprising subtracting the pixel gain correction fromX-ray image data collected using a digital X-ray detector to generate acorrected X-ray image.
 8. One or more non-transitory computer-readablemedia encoding one or more processor-executable routines, wherein theone or more routines, when executed by a processor, cause acts to beperformed, comprising: dividing pixels of X-ray image data into aplurality of pixel value regions based on a pixel value of each pixel,wherein each pixel value region has a different range of pixel values;generating calibrated X-ray image data for each pixel value region,wherein the respective calibrated X-ray image data for each pixel valueregion is generated from image data acquired during exposure to adifferent dose of radiation; calculating a gain slope for each pixelvalue region based on the calibrated X-ray image data; and calculating apixel gain correction for the pixels of the X-ray image data based on atleast one of the calculated gain slopes.
 9. The one or morenon-transitory computer-readable media of claim 8, wherein the one ormore routines, when executed by the processor, cause further acts to beperformed comprising: determining a pixel value boundary based on a userselection, wherein the pixel value boundary separates values of pixelswithin the respective pixel value region from other values of pixelswithin another pixel value region.
 10. The one or more non-transitorycomputer-readable media of claim 8, wherein the one or more routines,when executed by the processor, cause further acts to be performedcomprising: selecting the dose of radiation for each pixel value regionsuch that a receptor output at each radiation dose is substantiallycentral within the respective pixel value region.
 11. The one or morenon-transitory computer-readable media of claim 10, wherein the one ormore routines, when executed by the processor, cause further acts to beperformed comprising: acquiring a plurality of images from a digitalX-ray detector for each selected dose of radiation; and averaging theplurality of images acquired at each selected dose to determine anaverage gain calibration image for each pixel value region.
 12. The oneor more non-transitory computer-readable media of claim 8, wherein theone or more routines, when executed by the processor, cause further actsto be performed comprising: multiplying the gain slope by a relativepixel value to determine a relative pixel gain correction, wherein therelative pixel value is a pixel value within the respective pixel valueregion minus a lower pixel value boundary of the respective pixel valueregion; and adding the relative pixel gain correction to a full pixelgain correction of all other pixel value regions having pixel valuesbelow the lower pixel value boundary to generate the pixel gaincorrection.
 13. The one or more non-transitory computer-readable mediaof claim 8, wherein the one or more routines, when executed by theprocessor, cause further acts to be performed comprising: subtractingthe pixel gain correction from X-ray image data collected using adigital X-ray detector to generated a corrected X-ray image.
 14. Animaging system, comprising: a radiation source; a digital X-ray detectorconfigured to generate X-ray image data; control circuitry configured toacquire the X-ray image data from the digital X-ray detector; andprocessing circuitry configured to apply a gain correction to the X-rayimage data via a plurality of slope maps, wherein the processingcircuitry is configured to generate the slope maps by executing code toperform the acts of: dividing pixels of X-ray image data into aplurality of pixel value regions based on a pixel value of each pixel,wherein each pixel value region has a different range of pixel values;generating calibrated X-ray image data for each pixel value region,wherein the respective calibrated X-ray image data for each pixel valueregion is generated from image data acquired during exposure to adifferent dose of radiation; and calculating a gain slope for each pixelvalue region based on the calibrated X-ray image data.
 15. The imagingsystem of claim 14, wherein the processing circuitry is configured toapply the gain correction based on at least one of the calculated gainslopes.
 16. The imaging system of claim 14, wherein the digital X-raydetector comprises a complementary metal-oxide-semiconductor (CMOS)based detector.
 17. The imaging system of claim 14, wherein theprocessing circuitry is configured to generate the slope maps byexecuting code to perform the further acts of: determining a pixel valueboundary based on a user selection, wherein the pixel value boundaryseparates values of pixels within the respective pixel value region fromother values of pixels within another pixel value region.
 18. Theimaging system of claim 14, wherein the processing circuitry isconfigured to generate the slope maps by executing code to perform thefurther acts of: selecting the dose of radiation for each pixel valueregion such that a receptor output at each radiation dose issubstantially central within the respective pixel value region.
 19. Theimaging system of claim 14, wherein the processing circuitry isconfigured to generate the slope maps by executing code to perform thefurther acts of: acquiring a plurality of images from the digital X-raydetector for each selected dose of radiation; and averaging theplurality of images acquired at each selected dose to determine anaverage gain calibration image for each pixel value region.
 20. Theimaging system of claim 14, wherein the processing circuitry isconfigured to generate the slope maps by executing code to perform thefurther acts of: multiplying the gain slope by a relative pixel value todetermine a relative pixel gain correction, wherein the relative pixelvalue is a pixel value within the respective pixel value region minus alower pixel value boundary of the respective pixel value region; andadding the relative pixel gain correction to a full pixel gaincorrection of all other pixel value regions having pixel values belowthe lower pixel value boundary to generate the gain correction.
 21. Theimaging system of claim 14, wherein the processing circuitry isconfigured to apply the gain correction by subtracting the gaincorrection from X-ray image data collected using the digital X-raydetector to generate a corrected X-ray image.